1. Field of the Invention
The present invention relates to a valve drive mechanism for an engine which is variable in valve lift and has a valve lifter or tappet which selectively transmits rotation of different cams of different lift cams.
2. Description of Related Art
There has been known various valve drive mechanisms which can drive valves with variable valve lifts. For example in U.S. Pat. No. 5,287,830 a valve drive mechanism has a center tappet and a side tappet arranged coaxially with each other and couples them together by a hydraulically operated locking/unlocking pin for high speed engine operation with a high lift cam and uncoupled from one another by the hydraulically operated locking/unlocking pin for low speed engine operation with low lift cams. In Japanese Unexamined Patent Publication No. 10-141030 a cylindrically shaped tappet is divided into three parts in a rotational direction of cams. Further, in Japanese Unexamined Patent Publication No. 7-71213 a shim is divided into three parts.
The tappet disclosed in U.S. Pat. No. 5,287,830 comprises a cylindrical center tappet and a side tappet which coaxially surrounds the cylindrical center tappet. This cylindrical configuration of the tappet has restraints on the length of the center tappet as a cam follower. In order to avoid such a restraint, it is proposed to incorporate a center tappet having an elongated top. However, this alternative center tappet increases the height of the tappet. The tappet disclosed in Japanese Unexamined Patent Publication No. 10-141030 or Japanese Unexamined Patent Publication No. 7-71213 has the drawback that, since a circumferential outer wall at an edge of an interface of the side tappet with the center tappet causes contact slide on a wall of a tappet guide bore formed in a cylinder head in other words, since the center tappet is not subjected to a force by the cam, while the side tappet is driven by side cams, there occurs a rise in pressure between the side tappet and tappet guide bore, which results in uneven abrasion of the tappet and tappet guide.
It is therefore an object of the present invention to provide a valve drive mechanism which enables a large cam follower length of a tappet and lowers a force that is caused due to an inclination of the tappet and is exerted on a tappet guide from the tappet.
The above object of the present invention is accomplished by a valve drive mechanism including one center cam which has a center cam lobe per valve and a pair of side cams which have side cam lobes, respectively, different from the center cam lobe per valve and are arranged on a camshaft on opposite sides of the center cam in an axial direction of the camshaft, a generally cylindrically shaped tappet assembly which is movable in a direction of valve lift and comprises two mating parts, and locking/unlocking means for mechanically coupling the two mating parts together and uncoupling the two mating parts from each other so as to selectively transmit rotation of the center cam and the side cams as reciprocating movement to the valve. The valve drive mechanism comprises a center tappet, forming one of the two mating parts and driven by the center cam; which is formed with opposite circular-arcuate vertical side walls in a rotational direction of the camshaft, a side tappet, forming another one of the two mating parts and driven by the side cams, which is divided into two side tappet parts in the axial direction of the camshaft between which the center tappet is received for slide movement relative to the side tappet in said direction of valve lift and is formed at the side tappet parts with opposite circular-arcuate vertical end walls, respectively, such that the circular-arcuate vertical side walls of the center tappet and the circular-arcuate vertical end walls of the side tappet form a generally cylindrical configuration of the tappet assembly, and guide means for guiding the slide movement of the center tappet relative to the side tappet which comprises a vertical flat side wall extending continuously from each of opposite sides of each circular-arcuate end wall of the side tappet in the rotational direction of the camshaft and a vertical shroud extending continuously from each of opposite ends of each circular-arcuate side wall of the center tappet in the axial direction of the camshaft and forming thereon a vertical flat side surface. The vertical shroud at the vertical flat side surface is slidable on the vertical flat side wall so as thereby to guide the slide movement of the center tappet relative to the side tappet.
In the valve drive mechanism which preferably includes the center cam having a high lift cam lobe and the side cam having a low lift cam lobe, the two side tappet parts are joined by a connecting bridge at which the tappet assembly is engaged by a valve stem of the valve. This connecting bridge is formed with a spring receiving recess in which a return spring is received so as to force the center tappet to return when the center tappet slides relatively to the side tappet. Further, the connecting bridge may be provided with at least one oil spill port formed at a bottom of the spring receiving recess. A shim may be disposed between the connecting bridge and the valve stem.
The locking/unlocking means may preferably comprise guide bores which are formed in each the center tappet and each the side tappet part of the side tappet and are in alignment with one another in the direction of the rotational axis of camshaft, a locking/unlocking pin received for slide movement in the guide bore of the center tappet, a plunger received for slide movement in the guide bore of one of the two side tappet parts of the side tappet, a spring loaded receiver received for slide movement in the guide bore of another of the two side tappet parts of the side tappet, and an oil channel formed in the one side tappet part of the side tappet so as to communicate with the guide bore of the one of the two side tappet parts of the side tappet, through which hydraulic oil is introduced into and removed from the guide bore of the one side tappet part of the side tappet. The hydraulic oil is supplied into the guide bore of the one side tappet part of the side tappet through the oil channel so as to force the plunger and the locking/unlocking pin to slide against the spring loaded receiver and to partly enter the guide bores of the center tappet and the other side tappet part of the side tappet, respectively, thereby mechanically coupling the center tappet to the side tappet together and is removed from the guide bore of the one side tappet part of the side tappet through the oil channel so as to cause the plunger and the locking/unlocking pin to slide back by the spring loaded receiver, thereby mechanically uncoupling the center tappet from the side tappet. The locking/unlocking pin is preferably formed with a circumferential recess.
The tappet assembly may includes a stopper in the guide bore in which the plunger is received so as to limit the slide movement of the plunger in the guide bore and to close the guide bore at one end. In this case, the guide bore is communicated with the oil channel through a connecting oil channel.
The valve drive mechanism includes oil supply means comprising oil galleries which extend along the intake camshaft and the exhaust camshaft, respectively, branch oil channels which branch off from the oil galleries, respectively and extend between two tappet assemblies for twin intake valves and two tappet assemblies for twin exhaust valves for each cylinder, oil channels each of which is formed in an outer wall of the side tappet and is in communication with the branch oil channel. The plunger in the guide bore of the one side tappet part of the side tappet operates to bring the center tappet and the side tappet into a locked or mechanically coupled condition when pressure of hydraulic oil is supplied to the plunger from the oil gallery through the oil channel via the branch oil channel and into an unlocked or mechanically uncoupled condition when the pressure of hydraulic oil is removed from the plunger.
The branch oil channel preferably extends such as to partly overlap outer peripheries of the two tappet assemblies for the twin intake valves or the twin exhaust valves, and the oil channel has a length sufficient to remain communicated with the branch oil channel during up and down movement of the tappet assembly.
The branch oil channel may be formed by drilling a cylinder head to the oil gallery from one side of the cylinder head and plugged at the one side of the cylinder head.
The valve drive mechanism may further comprise a member operative to prevent the tappet assembly from turning relative to the cylinder head during installing the tappet assembly in the valve drive mechanism. The member is provided on an outer wall of the side tappet at one of opposite sides of the tappet assembly remote from the branch oil chamber.
The valve drive mechanism may further comprises retaining means provided between the center tappet and the side tappet for preventing the center tappet from moving up beyond a top of the side tappet by the return spring and however for allowing down movement of the center tappet with respect to the side tappet against the return spring. Specifically, the retaining means comprises a retaining pin extending between the center tappet and the side tappet, a supporting bore in which the retaining pin is removably received and a limiting recess engageable with the retaining pin which limits the down movement of the center tappet, the supporting bore being formed in either one of the center tappet and the side tappet and the limiting recess being formed in another one of the center tappet and the side tappet.
According to the valve drive mechanism, the tappet assembly has the vertical shroud which extends, preferably along almost the entire vertical length of the center tappet, continuously from each of opposite sides of each circular-arcuate vertical side wall of the center tappet in the axial direction of the camshaft, slide movement of the center tappet relative to the side tappet is guided by the vertical shrouds sliding on the vertical flat side wall of the side tappet, respectively. This structure of the tappet assembly enables a large cam follower length of the tappet assembly. In addition, the tappet assembly thus structured disperses a force, which presses the side tappet against the guide wall of the tappet guide, toward the center tappet through the vertical shrouds while the side tappet is driven by the side cams, so that the side tappet slides on the tappet guide through the outer wall of the center tappet that is perpendicular to a direction in which the force presses the side tappet against the wall of the tappet guide. As a result, there is no concentration of pressing force that occurs at circumferential outer edges of an interface with the center tappet in the conventional valve drive mechanisms. In addition, the force that is caused due to an inclination of the tappet and is exerted on the tappet guide from the tappet is lowered.
The valve drive mechanism has the cam arrangement in which the high lift center cam is disposed between the low lift side cams enables a large cam follower length of the tappet assembly. This cam arrangement is quite advantageous to high lift operation. In addition to the cam arrangement, the valve drive mechanism has the side tappet structure in which the two side tappet parts are joined by the connecting bridge engageable with the valve stem and the return spring is received in the recess formed in the connecting bridge so as to force the center tappet to return. This side tappet arrangement keeps the center tappet ridden on the center cam while the center tappet is uncoupled from the side tappet. This prevents an occurrence of rattling noises due to repeated collisions of the center tappet with the center cam during floating action of the center tappet and, in addition, provides the tappet assembly with compactness.
The locking/unlocking means that comprise guide bores formed in the center tappet and the side tappet, a locking/unlocking pin received for slide movement in the guide bore of the center tappet, a plunger received for slide movement in the guide bore of one of the two side tappet parts, a spring loaded receiver received for slide movement in the guide bore of another one of the two side tappet parts, and an oil channel formed in the one side tappet part so as to communicate with the guide bore of the one side tappet part through which hydraulic oil is introduced into and removed from the guide bore of the one side tappet part. This locking/unlocking means operates such that, when hydraulic oil is supplied into the guide bore of the one side tappet part through the oil channel, the locking/unlocking means forces the plunger and the locking/unlocking pin to slide against the spring loaded receiver and to partly enter the guide bores of the center tappet and the other side tappet part, respectively, thereby mechanically coupling the center tappet to the side tappet together and, when the hydraulic oil is removed from the guide bore of the one side tappet part through the oil channel, the locking/unlocking means causes the plunger and the locking/unlocking pin to slide back by the spring loaded receiver, thereby mechanically uncoupling the center tappet from the side tappet. This hydraulically operated mechanism of the locking/unlocking means can couple the center tappet to the side tappet together in a state where the engine operates at a high speed and, in consequence, a high hydraulic pressure is provided assuredly. This prevents an occurrence of unstable mechanical coupling of the center tappet to the side tappet due to an insufficient hydraulic pressure.
The locking/unlocking pin formed with a circumferential recess decreases an area of contact surface with the guide bore, so as to lower frictional resistance between the locking/unlocking pin and the guide bore.
The valve drive mechanism includes the oil channel arrangement for the tappet assembly which comprises the oil galleries extending along the intake camshaft and the exhaust camshaft, respectively, branch oil channels branching off from the oil galleries, respectively and extending between the two adjacent tappet assemblies for the twin intake valves and the two adjacent tappet assemblies for the twin exhaust valves for each cylinder, oil channels each of which is formed in an outer wall of the side tappet and is in communication with the branch oil channel. Further, in the oil channel arrangement, the branch oil channel extends such as to partly overlap outer peripheries of the two tappet assemblies for the twin intake valves or the twin exhaust valves, and the oil channel has a length sufficient to remain communicated with the branch oil channel during up and down movement of the tappet assembly. The oil channel arrangement has one branch oil channel used commonly to both the two adjacent tappet assemblies. This avoids drilling the branch oil channel per the tappet guide, which leads to a reduction in man-hour for forming the branch oil channel. In addition, the oil channel arrangement is easily formed.